Separation of solutes in gaseous solvents

ABSTRACT

An apparatus capable of being used for the separation of solutes dissolved in a gaseous solvent is described. In one aspect, the apparatus includes a bed of sorbent in a container, a device which allows the gaseous solvent to pass through the bed of sorbent in a first direction, a device which allows the pressure of the gas to increase and/or the temperature of the gas to decrease, and a device which allows the gaseous solvent to pass through the bed of sorbent in a second direction. In another aspect, the device is capable of allowing the bed of sorbent to be contaminated by the solute and also be regenerated in situ.

This application is a divisional application of application Ser. No.08/027,890 filed on Mar. 8, 1993, now U.S. Pat. No. 5,470,377, issuedNov. 23, 1995.

BACKGROUND OF THE INVENTION

This invention relates to a method and apparatus for the separation ofsolutes dissolved in gaseous solvents. In particular this inventionrelates to the separation and recovery of one or more solutes dissolvedin high pressure gases and the recovery of the purified gas at highpressure. For the purposes of this invention, a gaseous solution isconsidered to be a solution where the solvent or the solution hassignificant compressibility.

At low pressure gases are poor solvents, and the potential solubility ofa potential solute is limited to its volatility. For example themoisture content of air is limited by the vapor pressure of water at thedew point, which is the temperature at which the vapor pressure of waterequals the partial pressure of water in the gas. At high pressure gasescan have significant solvent action even for nonvolatile substances, forexample high pressure steam can dissolve silica at temperatures wherethe vapor pressure of silica is negligible. See FIG. 4.

The solvent action of a particular gas has a strong dependance on gasdensity as influenced by pressure or temperature, where a particular gasat high density can dissolve more solute than the same gas at a lowerdensity. This strong effect of density or pressure on solute solubilityis well known and is used in the field of supercritical fluidextraction, which is well described by Krukonis and McHugh, in theirtext "Supercritical Fluid Extraction" Butterworth Publishers 1986, andby Zosel U.S. Pat. No. 3,969,196.

In a typical SCF process a solute is extracted from a substrate using ahigh density, high pressure gaseous solvent, the solvent is then reducedin pressure (and hence density) to where the concentration of the solutenow exceeds the new solubility and the solute precipitates out and canbe mechanically recovered from the gaseous solvent which now can becompressed and reused. This typical process is well described byKrukonis and McHugh and earlier by Zosel.

There are drawbacks to this type of density reduction by pressurereduction, and also with the corresponding density reduction bytemperature increase process, namely:

1. Significant solute remains dissolved unless the pressure is taken tovery low levels, necessitating large compression costs for solventreuse.

2. Formation of 2 phases in pressure reduction or temperature adjustmentequipment can lead to erosion, or reduction of heat transfercoefficients.

There are also cases where a reduction in the pressure of the highpressure gas is not feasible. For example in the production of highpressure steam. Silica is an ubiquitous contaminant in ground andsurface water. In a boiler the high pressure steam dissolves some silicaand carries it through the system until the turbine where the steam isexpanded to low pressure. The silica becomes less soluble in the lowpressure steam and precipitates as a solid inside the turbine causingdegradation of performance. The solubility of silica and other mineralsin steam necessitates extreme purity in boiler feed water and precludesthe open cycle use of geothermal steam or steam from supercritical wateroxidation as described by Modell U.S. Pat. No. 4,338,199 or DickinsonU.S. Pat. No. 4,292,953. Work is only extracted from the steam byexpansion to a lower pressure where silica precipitation is inevitable.

Petroleum can be recovered from underground formations by using highpressure carbon dioxide as a solvent. The expansion, phase separationand recompression of the gas can be expensive and may not recover manyof the lighter hydrocarbons, unless expansion to a very low pressure isutilized, which increases compression costs.

SCF processing is used for many separation applications, mostlyinvolving high value materials like specialty food stuffs, such asdecaffeinated coffee, where incidentally the solute caffeine istypically not recovered, but is instead irreversibly adsorbed ontoactivated carbon.

A major cost in SCF processing is the energy needed to compress thefluid so as to restore its solvent properties after expansion to removesolute. A sorbent such as activated carbon can be used to remove solutewhile avoiding a pressure reduction. Where the desired product is thesolute, for example in oil seed extraction or for spice extraction thena pressure change and the resulting energy cost is unavoidable.

Work is a force times a distance. The work of compression of a fluiddepends to a great extent on the compressibility of the fluid and thevolume change that occurs during compression. Liquids have lowcompressibilities and require little work to raise their pressure. Gaseson the other hand because of their compressibility require much morework to compress. Compression of a dense gas through a modest densitychange can require less work than compression of the low pressure gasthrough a larger volume change, even though the pressure change, or themaximum pressure may be lower.

High pressure gases can dissolve solutes in processes where this effectis undesirable. For example in the compression of gases the lubricantsfrom the compressor can contaminate the gas. In the case of carbondioxide to be used for semiconductor cleaning, separation of lubricantsor other low volatility contaminants entails distillation. Distillationof carbon dioxide must occur in the subcritical two phase region. Toobtain supercritical carbon dioxide the fluid must then be compressedand heated. In the case where the solute is a solid, distillationresults in the formation of solid in the distillation column which cancause plugging and inefficient operation. Similarly fluids used incryogenic refrigeration cycles must be cleaned of solutes prior toexpansion so as to avoid solid formation and concentration of oil in theevaporator and inadequate lubrication of the compressor.

Supercritical fluid chromatography in particular when using pressureprogramming can be used to separate a great many compounds, especiallyhydrocarbons on an analytical scale. Extremely pure fluids are needed,and compressor lubricants cannot be easily removed.

OBJECTS OF THE INVENTION

1. To provide a process where a solute can be separated and recoveredfrom a gaseous solvent.

2. To provide a process where a gaseous solvent can be purified to anarbitrarily high level.

3. To provide a process where a solute can be separated from a gaseoussolvent to arbitrarily high levels.

4. To provide a process where different solutes with differentsolubility properties can be separated from each other while dissolvedin a gaseous solvent.

5. To remove silica and other minerals from impure steam to allow itsuse in processes requiring pure steam such as turbines.

6. To purify a SCF solvent for reuse.

7. To provide a separation process which can avoid passing a two phasemixture through a pressure changing device or a temperature changingdevice.

THEORY OF OPERATION

Prior methods for removing solutes from gaseous solvents have eitherinvolved the change of pressure or temperature to form a two phasemixture, or the use of a sorbent to irreversibly sequester the solute.

There is a body of art concerned with a similar problem, that ofseparating gases from each other. For example when ambient air iscompressed the moisture present in the ambient air contaminates thecompressed air and for many uses this moisture must be removed. Dropletsof water can be mechanically filtered, but water vapor presents adifferent problem. Refrigerated air driers cool the air and so lower thedew point of the dried air. Beds of sorbent or desiccant can be used,and these beds can be regenerated by heating.

Another method is known as pressure swing adsorption, or when applied toair drying, oxygen recovery from air, and separation of hydrocarbons"heatless drying", or "heatless fractionation" (S. Skarrstrom et al. inU.S. Pat. Nos. 3,069,830, 3,237,377 and 2,944,627 and Kant et al. inU.S. Pat. No. 3,237,379). In the drying process water vapor in highpressure (150 psia) air is adsorbed onto a sorbent in a bed, and thenthe bed is depressurized and a volume of dry air at low pressure ispassed through the bed in the opposite direction where this regeneratingair now removes the adsorbed moisture from the desiccant. This processworks because the vapor pressure of water sorbed onto the desiccant is(nearly) independent of the pressure of the air also in contact with thedesiccant. Ten volumes of air at 150 psia hold as much water vapor asten volumes of air at 15 psia. In operation ten volumes of air at 150psia can be dried to a very high level in a bed of desiccant and thenthe pressure is reduced and perhaps twenty volumes of air at 15 psia isback flowed through the bed and used for bed regeneration. This stillleaves eight volumes of dried air at 150 psia for use.

Heatless drying and other forms of pressure swing adsorption areconcerned with the separation of one type of gas from another where thegases behave as an ideal mixture and the presence or absence of one gasdoes not greatly affect the sorption of the other gases. In theseparation of water from air, the air does not greatly affect thepartial pressure of water, and air at reduced pressure is used forregeneration. The current state of the art in pressure swing adsorptionis well described in an article by R. V. Jasra et al "Separation ofGases by Pressure Swing Adsorption", Separation Science and Technology,26(7), pp. 885-930, 1991. In all processes the sorbent is loaded at highpressure and desorbed at low pressure.

In the context of this disclosure a gaseous solvent is considered to bea solvent with a significant compressibility, and a solute or anongaseous solute is a species whose thermodynamic chemical potentialdecreases with increased density of the gaseous solvent or solution.

In the process of this invention a gaseous solvent is to be purified. Ina gaseous solvent, the solubility of a solute increases with increaseddensity of the solvent. The thermodynamic chemical potential of a puresolid or liquid is a very weak function of pressure, and so in asaturated solution where the gaseous solution is in equilibrium withpure solute the chemical potential of the solute in solution is alsoindependent of pressure if the solution remains saturated. The chemicalpotential of a species in solution not at saturation is to a firstapproximation proportional to the fractional degree of saturation. Whenthere is a difference in chemical potential of a species between twophases the species moves from regions of higher chemical potential toregions of lower chemical potential until the chemical potential isuniform. Thus in an under saturated solution in contact with pure solutethe solute dissolves until the solution is saturated. Similarly in asupersaturated solution solute precipitates.

When a mixture of gaseous species is isothermally compressed, thechemical potential of each of the gaseous species increases. In the caseof water vapor in air, the chemical potential increases until it exceedsthat of liquid water and liquid water then precipitates. The vaporpressure of water (and the chemical potential) remains constant as totalpressure increases and more water precipitates.

When a gaseous solution is isothermally compressed the chemicalpotential of the solvent gas increases, but the chemical potential ofthe nongaseous solute decreases. The solubility of a nongaseous soluteincreases with increased pressure and more solute must then dissolve tomaintain the chemical potential of the solution. This phenomena isuniversally observed in the near super critical region and is commonlyexploited during SCF processing. In FIG. 4 the change in solubility ofsilica in water can be seen to increase with pressure along the variousisotherms. This behavior is also observed in the subcritical gaseousregion, although it is less pronounced.

If a mixture of air and water vapor is maintained in equilibrium with adesiccant, then the chemical potential of water on the desiccant and inthe gas are equal. If the gas is isothermally compressed water will moveonto the desiccant. If the gas is isothermally expanded water will moveoff the desiccant.

If a solution of a gaseous solvent and a solute is maintained inequilibrium with a sorbent then the chemical potential of the solute onthe sorbent and in the solution are equal. If the gaseous solution isisothermally compressed then the chemical potential of the solute insolution decreases and solute moves off the sorbent and into solution.If the solution is expanded, then solute moves out of solution and ontothe sorbent. This behavior is opposite that observed with gaseousmixtures, and opposite that observed and utilized in pressure swingadsorption or heatless drying. Just as pressure swing adsorption is aform of parametric pumping, so is the process of this invention. Soluteis sorbed at low pressure and is desorbed at high pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a first embodiment of an apparatus forpracticing a method of separating and recovering one or more solutesdissolved in a gaseous solvent and for recovering a purified gas;

FIG. 2 is a schematic of a second embodiment of an apparatus forpracticing the method according to the present invention;

FIG. 3 is a schematic of a third embodiment that can be used forpracticing a method of the present invention;

FIG. 4 is a graph of the solubility of silica in water; and

FIG. 5 is a schematic of a fourth embodiment for practicing a methodaccording to the present invention.

DESCRIPTION OF THE INVENTION

A process complementary to pressure swing adsorption is described wherea gaseous solution is purified by adsorption onto a sorbent bed, andthen by increasing the pressure and hence the density and reversing theflow the sorbent bed can be regenerated.

Consider the apparatus of FIG. 1 processing a feed containing carbondioxide as the solvent and naphthalene as the solute at a temperature of35° C. At 1100 psi the solubility is 0.2%, while at 1300 psi thesolubility is 2.0%, all on a weight basis.

If a feed containing 0.1% naphthalene at 1100 psi (50% of saturation) iscaused to flow in the bottom (81) of and through adsorption column 11which is packed with a suitable sorbent for example activated carbon,the naphthalene will adsorb on the sorbent and pure carbon dioxide willleave at the top (71). If 1000 kg of mixture is passed through, then 999kg of CO2 will be recovered and 1 kg of naphthalene will remain in thebed. At the base 81 of the bed where the feed first contacts thesorbent, the sorbent becomes loaded until the chemical potential fornaphthalene is 50% of saturation.

The feed flow is now diverted by closing valve 51 and opening valve 52to adsorption column 12, and pump 21 is activated to pressurizeadsorption columns 11 and 13 through valve 55 from 1100 to 1300 psi. Thesolubility of naphthalene now increases to 2.0%, but the chemicalpotential of the naphthalene on the sorbent at the base 81 of adsorptioncolumn 11 does not change with pressure, and so is still 50% ofsaturation. The saturation value is now 2.0%, so naphthalene desorbsfrom the sorbent until the fluid concentration at 81 is half thesaturation value, or 1.0%. Pump 21 Continues to pump, and fluid with1.0% naphthalene continues to move into column 13 until the 1 kg ofnaphthalene has been moved. This takes ideally 100 kg of CO2. This fluidpasses through column 13 and through the throttle valve 61 where thepressure is reduced to 1100 psi. The solubility at 1100 psi is only0.2%, so the excess naphthalene precipitates and saturated naphthalenein CO2 passes up through column 14. The excess naphthalene, or 0.8 kgcollects in vessel 31. Column 14 is loaded with sorbent, which adsorbsthe naphthalene, and at the base 84 becomes loaded until the chemicalpotential equals that at saturation. The partially depleted CO2 passesup through column 14 and joins with fresh feed prior to entering column12 where naphthalene is removed completely as in column 11.

When columns 12 and 14 are pressurized with pump 21 to 1300 psi, thesorbent at the base 84 of 14 is loaded to the chemical potential of asaturated solution and so the naphthalene content of fluid leaving 14 at84 will be 2.0%. That at the base 82 of 12 will be 1.0%. It is necessaryto remove 1.1 kg of naphthalene from column 12, which should takeideally 110 kg of CO2. 0.11 kg will be trapped in column 13 with 0.11being trapped in 11 and the remainder being collected as solid in vessel31.

This series of operation can be repeated many times by operating valves50 to 59 in the proper sequence, or by providing automatic controls todo so. Provided that the flow volumes are matched to the proper pressureand sufficient quantities of high pressure CO2 are used for regenerationarbitrary quantities of naphthalene-free CO2 can be recovered. Becausethe solubility and hence chemical potential increases 10 fold over thepressure change from 1100 to 1300 psi, the regeneration of the columnswith high pressure fluid requires (ideally) one tenth as much CO2 as inthe total feed stream, and the solute can in this case be recovered as apure material. Operationally it is easier to use an excess of fluid forregeneration so as to maintain a margin of safety in terms of productpurity, and to account for nonidealities and irreversibilities in theprocess. The flows used in this example are illustrative, and representthe ideal case.

In many cases a mixture of solutes is dissolved in a gaseous solvent andthe simultaneous recovery of all solutes is desired, for example in therecovery of petroleum with CO2 flooding. Carbon dioxide is aninexpensive fluid that at high pressure is a good solvent for mosthydrocarbons, and which lowers the viscosity of oil. Typically CO2 isfed under pressure into a petroleum formation where the residualhydrocarbons are dissolved and expanded by the CO2. The resultingmixture is removed from the formation. The CO2 has a much lowerviscosity than the CO2-oil mixture and so viscous fingering and theresulting nonplug flow through the formation can lead to dilution ofproduct with CO2. Many of the petroleum fractions that have value aremiscible with high pressure CO2, and recovery of petroleum and CO2 forreinjection can be difficult. Consider the apparatus in FIG. 2. In thiscase a fluid stream such as from a CO2 enhanced oil recovery process ora natural gas stream rich in natural gas liquids is fed into an expander22 to recover work. The expanded stream is then fed into columns, suchas in FIG. 1 where solutes are adsorbed onto a sorbent and separatedfrom the solvent gas. The expanded and cleaned fluid is then compressedin 21 using the work recovered in the prior expansion. A portion of thecompressed fluid is then used to regenerate the sorbent beds. Theexpander can in principle be operated in a singlephase region whereequipment is more reliable and efficient, yet solutes can be recoveredcompletely. Additional work is needed to recompress the regenerantfluid. This embodiment can be useful in petroleum recovery and refining,such as to increase the efficiency of for example propane deasphaltingor Residuum Oil Supercritical Extraction (the ROSE process), and innatural gas processing, or where it is necessary to remove a solute froma gaseous solvent. Krukonis and McHugh in their book describe manyexamples of gaseous solvent processes where the present invention wouldbe useful.

The process can be used to separate and recover toxic contaminants fromvarious substrates. For example SCF CO2 can be used to extract wasteoils, PCB's, aromatic hydrocarbons, and other soluble compounds fromcontaminated soil. The process of this invention allows thesecontaminants to be concentrated using the apparatus of FIG. 1 up tolevels where the contaminant can be incinerated or recycled, and thesolvent can be purified to very high levels so that reuse of the solventfor continued extraction can result in nearly complete removal ofcontaminants from the soil. The separation of the solute from thegaseous solvent occurs in the sorption column, and it is in principlenot necessary to ever form a 2 phase mixture, as is necessary inconventional SCF processing, as in the process of Eppig et al. (U.S.Pat. No. 4,434,028), where the solvent and solute form a two phasemixture for separation.

Regeneration of sorbents using supercritical fluids has been disclosedby Modell (U.S. Pat No. 4,061,566, 4,124,528, 4,147,624), and has beenwell studied and described by Krukonis and McHugh in their book. Thesediscuss the removal of adsorbates from sorbents where the sorbents werefirst used to adsorb solutes from an aqueous solution, and the gaseoussolvent used to desorb the solute and then is treated to make thegaseous solvent a nonsolvent for the solute under consideration. Whileactivated carbon can be regenerated with supercritical fluids, theadsorption of a solute can be substantial even in a dilute SCF solution.Krukonis presents data showing a loading of 0.2 g of alachlor (asynthetic pesticide) per gram of carbon at an alachlor concentration twoorders of magnitude below saturation. He thus shows that theregeneration expected by Modell is limited due to equilibrium effects.

The process of the current invention can be used to remove solutes froma gaseous solvent while at all times maintaining the fluid at conditionswhere it remains a solvent for the desired solute. The equilibriumeffects that limit the effectiveness of the process described by Modellare used in the process of the present invention to produce the desiredseparation.

The process can also be used for separating a mixture of solutes. InFIG. 1, the detector 42 monitors the stream from 11 and 12, and toproduce pure gaseous solvent the columns are regenerated when any soluteis detected. If it is desired to separate more strongly adsorbed speciesfrom less strongly adsorbed species, then the regeneration is delayeduntil the detector 42 detects the presence of the more strongly adsorbedspecies (which only starts to come out of the sorption column aftersubstantial quantities of the more weakly adsorbed species has), thensignificant amounts of the weakly held species can be recovered free ofthe more strongly held species in the fluid collected at the top of theapparatus. Similarly a detector 41 on the bottom can be used to detectthe presence of the weakly held species during regeneration, and todivert flow from the receiving vessel 31 into the adsorption columns 13or 14 for additional separation.

In many systems of two or more solutes, the solubility and hence theconcentration of one of the solutes can affect the solubility orchemical potential of the other solute. The use of so called"entrainers" can be used to increase solubility and increase extractionrate in SCF processing. When a solution with a mixture of solutes ispassed through an adsorption column the sorbent likely has a differentaffinity for each solute. The solute with the highest affinitypreferentially adsorbs first and solutes with lesser affinities areadsorbed later in the bed. In a long column this behavior results insequential breakthrough of the solutes. This same behavior is utilizedin chromatography where a mixture is injected at one end and theseparated compounds elute out sequentially at the other. There is agreat deal of art concerning the manufacture and selection of sorbentswith differential sorption affinities for different compounds so thatthe compounds may be separated and resolved in chromatography. Inchromatography, the fluid can be assumed to be in equilibrium with thepacking, and the sequential elution that is observed results from thesolute-sorbent equilibrium and the different times that differentsolutes spend in the mobile phase, or on the stationary phase.

Similarly the process of this invention can be used to separatedifferent solutes with different affinities for the sorbent used, andmethodologies for sorbent selection similar to those used inchromatographic art can be used.

In FIG. 3 an embodiment is shown that can be useful for removing silica,scale forming minerals and other impurities from impure steam, forexample geothermal steam, or supercritical water oxidation products, orsteam derived from wet air oxidation.

FIG. 4 shows the solubility of silica in water.

It can be seen that silica is appreciably soluble in high pressuresteam. In operation the steam is cleaned by passage through the sorptioncolumn. A portion of the fluid is withdrawn and passed through a heatexchanger to cool the fluid so as to protect the pump from extremes oftemperature.

An examination of the solubility curve of silica in water shows that inthe near supercritical region, the solubility decreases with increasingtemperature. In the apparatus of FIG. 3 some heat is removed prior tothe pump 21, so the regenerating fluid is at a higher pressure and at alower temperature. The FIG. 3 apparatus requires no external source ofpure water and will produce ultra pure steam from any source of lowpurity steam.

Instead of cooling, compressing and reheating a portion of the cleanedsteam, pure water could be heated and directly injected instead.Alternatively water at a lower temperature could be injected.

For example in FIG. 5 water from a SCW oxidizer 32 at 300 bar and 440°C. has a silica solubility of 0.01%. At 400 bar and 370° C. thesolubility is 0.1%. Thus 1/10 of the product steam would requirecompression for regeneration. The use of a lower temperature forregeneration reduces the amount of fluid needed, but the bed needs to beheated back to operating temperature to prevent transport of silicathrough the bed.

The cleaning of steam and the regeneration of the sorbent beds is mosteasily achieved in the near supercritical region. Mechanical work ismost efficiently extracted from steam that is much hotter. A SCWoxidation reactor 32 can be operated with less than stoichiometricoxygen so that the temperature is in the near supercritical region andcombustible organic remains. In SCW most organic materials are miscible,and have critical points lower than water. These organic species wouldnot be concentrated with the minerals, but would stay in the water richphase. Dissolved minerals can be removed and the steam still containingsubstantial fuel value can be combined with additional high pressure airin reactor 33, and clean, high temperature, high pressure steam can bemade without heat transfer surfaces in contact with impure streams. Thehigher pressure steam with a high mineral content can be used to preheatincoming water, air, or fuel by direct contact, and the minerals canthen be taken out with the other solids in the initial reactor, or in apost reactor solids separator 34.

In some cases modest chemical treatment can improve the treatability ofthe contaminated steam. For example H2S is commonly present ingeothermal steam. This can be oxidized to H2SO4 which is less volatile,and easier to remove with the present invention. After expansion of theclean steam the steam can be condensed to recover pure water.

Sorbent selection is important, but not critical. The sorbent must sorbthe solute, and must be resistant to the fluids and temperatures used.Sorbents such as zeolites, activated carbon, molecular sieves, (organicand inorganic), silica gel, activated alumina, and polymeric adsorptionresins are well known sorbents and can be used in the present invention.Prior art useful in the selection of suitable sorbents is thechromatographic art where solids as well as liquids on solid supportsare commonly used as sorbents. Liquids can be used as sorbents, butsubstantial complexity involving multiple pumps is encountered unlessthe liquid is held on a solid support. Also rapid mass transfer requireshigh interfacial area and short diffusion paths, especially in anyliquid phase, which is difficult to achieve without a solid support. Inthe case of steam purification, the fluid can be brine in some regionsof the phase diagram, and a solid substrate in other regions. When theprocess of this invention is practiced in the two phase region, then oneof the condensed phases can be utilized as the sorbent.

The chemical potential of the solute in the gaseous phase and thesorbent phase is determined by thermodynamic equilibrium effects. Theprocess of this invention works best when mass transfer is good betweenthe gas and the condensed phases, either the solid sorbent, or the fluidsorbent layer. This is conveniently achieved by using small sorbentparticles to minimize the diffusion path. Other well known means toaugment the mass transfer can be desirable. For example providing apulsatile flow so as to provide mixing while still maintaining plug flowis a well known technique for enhancing mass transfer in fluid-fluidcontacting. Another method is to use an electric field. Electric fieldgradients are well known for their ability to move regions of differentdielectric constant, and in a packed bed of a dielectric, theapplication of an external electric field will facilitate the capture ofdroplets by the solid. Liquid on a solid substrate is held there bycapillary action where the surface tension of the liquid stabilizes theposition of the droplet on the solid particle, especially when there issurface roughness. Some aspects of the use of electric fields for thecapture and filtering of droplets with electric fields are discussed inU.S. Pat. No. 4,786,387 and in the references discussed therein. In thatprocess, an electric field was used to shift the equilibrium as well asaugment the mass transfer. In the present process the equilibrium isshifted by the pressure change, and any electric field is only used toaugment the mass transfer, and by the process of dielectric filtrationreduce the entrainment of droplets of fluid containing impurities towardthe purified gas end. This can be particularly useful when high gasvelocities are desired and entrainment of liquid is a potential problem.

Desirable properties for a sorbent are:

1. reversible sorption

2. stability over many sorption-desorption cycles

3. selectivity (for solute-solute separation)

4. low cost

5. rapid mass transfer properties

High sorption capacity is not especially important. The fraction offluid that must be compressed and used for regeneration is a function ofchemical potential and thermodynamic equilibrium effects of thesolvent-solute system, and not a function of the sorbent capacity. Thesorbent capacity will affect the time between regeneration cycles, butnot the ratio of product to regenerant. A high sorbent capacity willallow longer cycles and smaller sorption beds. The cycle length may beimportant for other considerations, for example the fatigue life ofcycling pressure vessels. The volume of fluid to be compressed, the flowrate, the capital and operating cost of the compressor are nearlyindependent of cycle length (neglecting sorbent bed void volume). Alarge system would likely use a multiplicity of columns so thecompressor would be operating continuously, and the cycling achieved byvalves.

Relative sorbent capacities for different solutes, or selectivity isimportant for solute-solute separation, where the relative sorptioncapacity of a bed for a particular solute determines the breakthroughorder. Parametric pumping is a well known technique that has been wellstudied theoretically, and which theory allows for the rationalselection of sorbents and the estimation of separation per cycle.

In some cases the sorbent can be generated in situ. This is especiallyuseful where the sorbent deteriorates with time, or has someirreversible sorption characteristics, or where the active component ofthe sorbent system is generated in situ. For example in the clean up ofSCW oxidation products a high area ZrO2 can be used. This can begenerated in situ by injection of zirconium compounds that decompose orreact to form ZrO2 particles. These particles can trap a thin layer ofbrine or scale formed from salts dissolved in the steam. This brine andscale layer has a high affinity for minerals and can act just like asolid sorbent. This is analogous to the use of supported liquids ingas-liquid chromatography. Compounds can be added to facilitate brineformation of the proper chemistry. Similarly aerogels are often made byreacting gel forming compounds in a fluid and then removing the fluidabove its critical point. For metal oxide aerogels the typical solventis water. A soluble precursor is reacted in solution to form theinsoluble oxide. Water and reaction products are then removed to leavebehind the oxide aerogel, which with its high surface area can be aneffective sorbent. Similar processes are used to make zeolites which arewill known sorbents.

What is claimed is:
 1. An apparatus for separating at least one solutefrom a gaseous solvent under conditions where a chemical potential ofthe at least one solute dissolved in the gaseous solvent decreases as adensity of the gaseous solute increases, the apparatus comprising:a bedof sorbent in a container; means for passing the gaseous solventcontaining said at least one solute through said bed of sorbent in afirst direction so as to sorb said at least one solute from said gaseoussolvent onto said bed of sorbent to produce a purified gaseous solvent;means for acting upon a portion of the purified gaseous solvent toincrease a solvent capacity of the purified gaseous solvent for saidsolute, wherein the means for acting upon the portion of the purifiedgaseous solvent is selected from the group consisting of a pressureincreasing means and a temperature reducing means; and means for passingsaid portion of purified gaseous solvent through said bed of sorbent ina second direction opposite the first direction, to desorb said at leastone solute from said bed of sorbent to produce a gaseous solution ofsaid at least one solute in said portion of purified gaseous solvent. 2.The apparatus according to claim 1, further comprising detector means tomonitor said purified gaseous solvent.
 3. The apparatus according toclaim 1, further comprising detector means to monitor said gaseoussolution.
 4. The apparatus according to claim 1, further comprisingmeans for recovering said portion of purified gaseous solvent from thegaseous solution.
 5. The apparatus according to claim 1, wherein saidmeans for acting upon the portion of purified gaseous solvent comprisesa pressure increasing device.
 6. The apparatus according to claim 1,wherein said means for acting upon the portion of purified gaseoussolvent comprises a temperature decreasing device.
 7. The apparatusaccording to claim 1, wherein said bed of sorbent includes a sorbentselected from the group consisting of activated carbon, activatedalumina, zeolite, chromatographic support, metal oxide aerogels, silicagel, molecular sieves and mixtures thereof.
 8. An apparatus forseparating from each other at least two solutes dissolved in a gaseoussolvent under conditions where a chemical potential of a first solute ofthe at least two solutes decreases as a density of the gaseous solventincreases, the apparatus comprising:a bed of a sorbent in a container;means for passing said gaseous solvent containing the at least twosolutes through said bed of sorbent in a first direction so as to sorbthe first solute from said gaseous solvent onto said bed of sorbent,producing a purified gaseous solvent depleted in said first solute butstill containing a second solute; means for acting upon a portion of thepurified gaseous solvent to increase a solvent capacity of the portionof the purified gaseous solvent for said first solute so as to producean increased affinity gaseous solvent, wherein the means for acting uponthe portion of the purified gaseous solvent is selected from the groupconsisting of a pressure increasing means and a temperature reducingmeans; and means for passing the increased affinity gaseous solventthrough said bed of sorbent in a second direction opposite the firstdirection to desorb said first solute from said bed of sorbent, anddissolve the first solute in the increased affinity gaseous solvent toproduce a gaseous solution.
 9. The apparatus according to the claim 8,further comprising means for recovering said increased affinity gaseoussolvent from the gaseous solution.
 10. The apparatus according to claim8, further comprising means for recovering said second solute from saidpurified gaseous solvent.
 11. The apparatus according to claim 8,wherein said means for acting upon the portion of purified gaseoussolvent comprises a pressure increasing device.
 12. The apparatusaccording to claim 8, wherein said means for acting upon the portion ofpurified gaseous solvent comprises a temperature decreasing device. 13.The apparatus according to claim 8, wherein said bed of sorbent includesa sorbent selected from the group consisting of activated carbon,activated alumina, zeolite, chromatographic support, metal oxideaerogels, silica gel, molecular sieves and mixtures thereof.
 14. Theapparatus according to claim 8, further comprising means for monitoringsaid purified gaseous solvent.
 15. The apparatus according to claim 8,further comprising means for monitoring said second gaseous solution.16. An apparatus for purifying impure steam containing at least oneimpurity for use under conditions where a chemical potential of the atleast one impurity in the impure steam decreases as a density of steamincreases, the apparatus comprising:a bed of sorbent in a container;means for passing said impure steam through said bed of sorbent in afirst direction so as to remove by adsorption onto said sorbent the atleast one impurity in said impure steam; means for acting on a portionof pure water to produce pure steam having an increased density and anincreased affinity for said at least one impurity adsorbed onto said bedof sorbent; means for passing said pure steam having the increaseddensity and the increased affinity for the at least one impurity throughsaid bed of sorbent in a second direction opposite said first directionto desorb the at least one impurity adsorbed onto said bed of sorbent toproduce a second impure steam; and means for recovering the pure steamfrom the second impure steam.
 17. The apparatus according to claim 16,wherein said impure steam is generated using a SCW oxidation means. 18.The apparatus according to claim 16, wherein said purified steam isexpanded to recover work through a work generating means.
 19. Theapparatus according claim 16, wherein the purified steam is condensed torecover pure water with a steam condensing means.
 20. The apparatusaccording to claim 16, wherein the means for acting upon the portion ofpure water to produce pure steam with an increased affinity includesmeans for sequentially compressing the pure water to produce anincreased pressure water, and for heating the increased pressure purewater to produce the pure steam having the increased density andincreased affinity for the at least one impurity.
 21. An apparatus forseparating at least one solute from a gaseous solvent under conditionswhere a chemical potential of the at least one solute dissolved in thegaseous solvent decreases as a density of the gaseous solute increases,the apparatus comprising:a bed of sorbent in a container; means forpassing the gaseous solvent containing said at least one solute throughsaid bed of sorbent in a first direction so as to sorb said at least onesolute from said gaseous solvent onto said bed of sorbent to produce apurified gaseous solvent; means for acting upon a liquid solvent toproduce a gaseous solvent having an increased density and an increasedaffinity for the at least one solute; means for passing the gaseoussolvent having the increased density and the increased affinity for theat least one solute through said bed of sorbent in a second directionopposite the first direction, to desorb said at least one solute fromsaid bed of sorbent to produce a gaseous solution of said at least onesolute in said portion of purified gaseous solvent.
 22. The apparatusaccording to claim 21, wherein the means for acting upon the liquidsolvent includes means for sequentially compressing the liquid solventto produce an increased density liquid solvent, and for heating theincreased density liquid solvent to produce the gaseous solvent havingthe increased density and the increased affinity for the at least onesolute.
 23. An apparatus for separating from each other at least twosolutes dissolved in a gaseous solvent under conditions where a chemicalpotential of a first solute of the at least two solutes decreases as adensity of the gaseous solvent increases, the apparatus comprising:a bedof a sorbent in a container; means for passing said gaseous solventcontaining the at least two solutes through said bed of sorbent in afirst direction so as to sorb the first solute from said gaseous solventonto said bed of sorbent, producing a purified gaseous solvent depletedin said first solute but still containing a second solute; means foracting upon a liquid solvent to produce a gaseous solvent having anincreased density and an affinity for the first solute; means forpassing the gaseous solvent having the increased density and theincreased affinity for the second solute through said bed of sorbent ina second direction opposite the first direction to desorb said firstsolute from said bed of sorbent, and dissolve the first solute in theincreased affinity gaseous solvent to produce a gaseous solution. 24.The apparatus according to claim 23, wherein the means for acting uponthe liquid solvent includes means for sequentially compressing theliquid solvent to produce an increased density liquid solvent, and forheating the increased density liquid solvent to produce the gaseoussolvent having the increased density and the increased affinity for thefirst solute.
 25. An apparatus for purifying impure steam containing atleast one impurity for use under conditions where a chemical potentialof the at least one impurity in the impure steam decreases as a densityof steam increases, the apparatus comprising:a bed of sorbent in acontainer; means for passing said impure steam through said bed ofsorbent in a first direction so as to remove by adsorption onto saidsorbent the at least one impurity in said impure steam and so as toprovide a purified steam; means for acting on a portion of the purifiedsteam to produce pure steam with an increased affinity for said at leastone impurity adsorbed onto said bed of sorbent, wherein the means foracting on the purified steam to produce pure steam with the increasedaffinity is selected from the group consisting of a temperature reducingmeans and a pressure increasing means; means for passing said increasedaffinity pure steam through said bed of sorbent in a second directionopposite said first direction to desorb the at least one impurityadsorbed onto said bed of sorbent to produce a second impure steam; andmeans for recovering the pure steam from the second impure steam.